Draughty Sash Windows: Where the Air Actually Gets In

The four leak paths in a box sash

First, the meeting rail where the two sashes overlap — decades of paint build-up and shrinkage leave a horizontal slot right at head height. Second, the staff and parting beads, where the sash slides. Third, the pulley stiles and the weight pockets behind them, which open into an uninsulated cavity. Fourth, the joint between the frame and the masonry reveal, often hidden behind decorative architrave.

• Meeting rail overlap
• Staff and parting bead runs
• Pulley stiles and weight pockets
• Frame-to-masonry perimeter

Why brush draughtproofing only gets you part way

Routed-in brush pile seals the bead runs and meeting rail well and will noticeably calm a windy night. What it cannot seal are the weight pockets, and it does nothing for the radiant cold of a single 4 mm pane — you stop feeling moving air but still feel the cold falling off the glass. Sash draughtproofing on its own is a partial answer.

What a room-side panel does instead

A secondary unit is sealed to the plaster reveal, inboard of every one of those four leak paths. Whatever leaks through the sash now leaks into a contained cavity rather than into the room, and the room-side glass is warm enough to stop the cold downdraught. The original sash keeps working — vertical sliders let you open both layers together in summer.

Keeping the sash operational

We template the reveal so the secondary unit sits behind the architrave line without fouling the sash travel, and we specify vertical sliders on box sashes as standard so you can still throw the window open. Every panel lifts out or slides clear for cleaning both faces of both panes.